Computerized tomography image reconstruction

ABSTRACT

Systems and methods include coordinated (KV) and megaelectronvolt (MV) computerized tomography (CT) imaging. KV and MV data are combined using a normalization process in order to generate CT images. The resulting CT images can include an improved signal to noise ratio in comparison to CT images generated using either KV or MV imaging alone. The coordinated KV and MV imaging process may be accomplished in significantly less time than using KV or MV imaging alone. This time savings has advantages in treatment verification. The MV projections are optionally generated using MV x-rays configured for x-ray treatment. In these cases the combined projections will reflect the treatment volume.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional patentapplication Ser. No. 11/193,160, filed Jul. 29, 2005, now U.S. Pat. No.7,453,976, the disclosure of which is incorporated herein by reference,which claims priority to and the benefit of U.S. Provisional PatentApplication Ser. No. 60/682,170, entitled “A Technique for On-Board CTReconstruction Using Both Kilovoltage and Megavoltage Beam Projectionsfor 3-D Treatment Verification,” filed May 17, 2005, now lapsed, thedisclosure of which is incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The invention is in the field of medical imaging and more specificallyin the field of computerized tomography.

2. Related Art

Computerized tomography (CT) is an imaging technique wherein x-rays areused to obtain two-dimensional projection images at a variety ofdifferent angles around a target being examined. Computer techniques arethen used to generate a three-dimensional representation of the targetby combining the two-dimensional projection images. Thethree-dimensional representation can be viewed, sliced and rotated by auser.

CT systems can generally be characterized by the energies of the x-raysused, such as kilovoltage (kV) and megavoltage (MV) imaging. In kVimaging, x-rays with energies in the kiloelectronvolt range aregenerated and detected. In MV imaging, x-rays with energies in themegaelectronvolt range are generated and detected. Each of these typesof imaging has advantages and disadvantages. For example, kV imaging maybe subject to interference from tooth fillings and MV imaging may causeradiation damage to the DNA of living cells. MV imaging is sometimesused therapeutically as a cancer treatment.

In diagnostic CT imaging hundreds of two-dimensional projection imagesare recorded as an x-ray source and detector are rotated around thetarget. The quality of the final three-dimensional representation isdependent on the number of two-dimensional projection images used togenerate the three-dimensional representation. The time required torecord hundreds of two-dimensional projection images can be a problemwhen the target is a patient because the patient must stay still duringthe imaging process. Typically, diagnostic CT imaging is performed usingkV imaging because of the danger to the patient of using MV x-rays togenerate so many projection images.

One therapeutic use of MV x-rays is referred to as intensity-modulatedradiation therapy (IMRT). IMRT enables caregivers to deliver anextremely conformal dose of high energy x-rays to a well definedtreatment volume while minimizing radiation damage to nearby organs andtissues. The success of IMRT is largely dependent on the accuracy ofpatient positioning and target localization. Therefore, it is importantto have an efficient and effective method to confirm the position of thepatient and the target volume within the patient. Without confirmationof the position of the target volume, the x-ray dose may harm healthytissue and miss the tissue requiring treatment. In many situations avolume that is larger than the volume of tissue to be treated is exposedto high energy x-rays in order to compensate for errors in patientpositioning, organ motion, and target localization uncertainties. Thisresults in an undesirable exposure of healthy tissue to these x-rays.

There is, therefore, a need for improved methods of imaging that providegreater speed of analysis and greater accuracy for target localization.

SUMMARY

Systems and methods including more than one x-ray source and detectorcombination are used to generate separate two-dimensional projectionimages. Each source/detector combination is moved relative to the targetin order to create a series of overlapping projection images. Byoperating each source/detector combination in parallel in time, the timerequired to generate a series of two-dimensional projection images canbe substantially reduced. In comparison to the time requirements andresulting resolution of the prior art, this time savings can be used togenerate a three-dimensional representation in a shorter time and/or togenerate a three-dimensional representation with better resolution inthe same time.

In various embodiments of the invention, the more than one x-ray sourcesare configured to generate x-rays in different energy ranges. Forexample, in some embodiments, one source/detector combination is used togenerate projection images using kV x-rays while another source/detectorcombination is used to generate projection images using MV x-rays. Thesesource/detector combinations may operate in parallel. Thus, twodifferent projection images can be obtained at the same time. Theprojection images generated by one source/detector combination areoptionally scaled such that they can be combined with projection imagesgenerated by the other source/detector combination. The combinedprojection images are then used to generate three-dimensionalrepresentations of a target.

The three-dimensional representations may be used for targetlocalization. For example, in some instances, therapeutic MV x-rays areused to provide medical treatment while at the same time generating MVprojection images of a target area. These MV projection images arecombined with kV projection images recorded in parallel with the MVprojection images, in order to generate a three-dimensionalrepresentation that can be used for real-time target localization.

(copies of independent claims go herexxx)

BRIEF DESCRIPTION OF THE VARIOUS VIEWS OF THE DRAWING

FIG. 1 is a block diagram of an imaging system, according to variousembodiments of the invention;

FIG. 2 is a photograph showing of parts of the imaging system of FIG. 1,according to one embodiment of the invention;

FIG. 3 is a flowchart illustrating methods of generating athree-dimensional representation of a target using the imaging system ofFIG. 1, according to various embodiments of the invention;

FIG. 4 is a graphical representation of projection angles used togenerate projection images, according to various embodiments of theinvention;

FIG. 5 is a scatter plot of kV and MV conversion parameters, accordingto one embodiment of the invention;

FIG. 6 illustrates the conversion of MV projection image data to kVprojection image data, according to one embodiment of the invention;

FIGS. 7A and 7B are CT images obtained using MV and kV imaging,respectively, according to one embodiment of the invention;

FIGS. 8A-8F are CT images obtained using various imaging modes,according to one embodiment of the invention; and

FIGS. 9A-9C include CT images generated using various imaging modes,according to one embodiment of the invention.

DETAILED DESCRIPTION

Typical embodiments of the invention include two or more x-raysource/detector combinations. Each of these source/detector combinationsincludes an x-ray source and an x-ray detector. The x-ray source isconfigured to generate x-rays and direct them toward the associatedx-ray detector. The x-ray detector is configured to detect receivedx-rays in a spatially resolved manner and to generate resultingprojection image data. Typically, a target is placed between the x-raysource and x-ray detector for analysis. The detected x-rays are,therefore, representative of a projection of the target onto the x-raydetector. In medical applications, the target is often part of apatient.

Each of the source/detector combinations is configured to image anoverlapping volume within the target. For example, in some embodiments,the path of x-rays from a center of an x-ray source to a center of anx-ray detector can be represented by a beam axis, and eachsource/detector combination is configured such that their respectivebeam axes intersect in a target region.

Each source/detector combination is optionally operated in parallel togenerate projection image data. This parallel operation can be used toreduce the time required to generate a three-dimensional representationof a target and/or increase the resolution of the resultingthree-dimensional representation. In some embodiments, paralleloperation includes simultaneous generation of x-rays. In someembodiments, parallel operation includes generation of x-rays by onex-ray source while the detector of another source/detector combinationis involved in a data transfer process. In these embodiments, thegeneration of x-rays and data transfer processes may alternate betweensource/detector combinations. In alternative embodiments, eachsource/detector combination is operated in series.

In some embodiments, a first source/detector combination is configuredto generate x-rays in one energy range while a second source/detectorcombination is configured to generate x-rays in another energy range.For example, the first source/detector combination can be configured togenerate projection image data using kV x-rays while the secondsource/detector combination can be configured to generate projectionimage data using MV x-rays. As described further herein, projectionimage data generated using one energy range is optionally scaled suchthat it can be combined with projection image data generated usinganother energy range in order to generate a three-dimensionalrepresentation.

In some embodiments, the use of more than one energy range to generate athree-dimensional representation allows for reduction of disadvantagesassociated with a particular energy range. For example, the use of MVx-rays may reduce the generation of artifacts seen in kV only data, andthe use of kV x-rays may reduce injury to tissue surrounding the targetvolume that would be caused by MV x-rays.

FIG. 1 is a block diagram of an Imaging System 100 configured togenerate a three-dimensional representation of a target positioned in aTarget Region 105, and optionally configured to provide a therapeuticdose of x-rays to a treatment region within Target Region 105. Theinstance of Imaging System 100 illustrated in FIG. 1 includes twosource/detector combinations. A first combination, referred to as the MVdetector/source combination, is configured to generate projection imagedata using MV x-rays. The MV detector/source combination includes a MVX-Ray Source 110 and a MV X-Ray Detector 115 disposed such that theirassociated beam axis passes through Target Region 105. A secondcombination, referred to as the kV detector/source combination, isconfigured to generate projection image data using kV x-rays. The kVdetector/source combination includes a kV X-Ray Source 120 and a kVX-Ray Detector 125 disposed such that their associated beam axisintersects the beam axis of the MV detector/source combination withinTarget Region 105.

In typical embodiments, MV X-Ray Source 110, MV X-Ray Detector 115, kVX-Ray Source 120 and kV X-Ray Detector 125 are coupled to a movableGantry 130. Gantry 130 is configured to rotate around Target Region 105under the control of Motor 135. In alternative embodiments, Motor 135 isconfigured to rotate a target within Target Region 105. In theseembodiments, Gantry 130 is optionally stationary. The kV detector/sourcecombination is optionally rotated around Target Region 105 independentof the MV detector/source combination.

Imaging System 100 further includes a Control Logic 140 configured foroperating Motor 135, MV X-Ray Source 110, MV X-Ray Detector 115, kVX-Ray Source 120, and kV X-Ray Detector 125. Control Logic 140 typicallyincludes a processor and memory configured for storing projection imagedata. Control Logic 140 is further configured to control an optionalConversion Logic 145, a CT Construction Logic 150 and an optionalDisplay 155.

Conversion Logic 145 is configured to scale projection image datareceived from MV X-Ray Detector 115 such that the scaled data can becombined with projection image data received from kV X-Ray Detector 125.In alternative embodiments, Conversion Logic 145 is configured to scaleprojection image data received from kV X-Ray Detector 125 such that thescaled projection image data can be combined with projection image datareceived from MV X-Ray Detector 115. The scaling can include logarithmicfunctions known in the art. The operation of Conversion Logic 145 isdescribed further elsewhere herein.

CT Construction Logic 150 is configured to construct a three-dimensionalrepresentation of a target using two-dimensional projection images ofthe target generated using both MV X-Ray Detector 115 and kV X-RayDetector 125. The construction process can be performed using severalalternative construction techniques known in the art. For example, invarious embodiments, the filtered back-projection technique ormulti-level scheme algebraic reconstruction technique (MLS-ART) are usedfor construction of a three-dimensional representation.

Optional Display 155 is configured for viewing various data generatedusing MV X-Ray Detector 115 and kV X-Ray detector 125, and for viewingthree-dimensional representations of a target constructed using CTConstruction Logic 150. Typically, three-dimensional representations areviewed as cross-sections of the three-dimensional representation. Thesecross-sections are referred to as CT images.

FIG. 2 is a photograph showing of parts of Imaging System 110, accordingto one embodiment of the invention. The MV source/detector combinationand kV source/detector combination are positioned such that theirrespective beam lines intersect at approximately a right angle in TargetRegion 105. FIG. 2 shows an experimental instance of a Target 210positioned within Target Region 105. As is discussed further herein,this experimental instance of Target 210 includes a simulation of partof a human head and a device used to study resolution in x-ray imaging.In practice, the instance of Target 210 shown in FIG. 2 is typicallyreplaced by a patient.

FIG. 3 is a flowchart illustrating methods of generating athree-dimensional representation of Target 210 using Imaging System 100,according to various embodiments of the invention. In this method the kVsource/detector combination and the MV source/detector combination areused to generate separate projection image data. Optionally, a first setof projection image data generated using one of these source/detectorcombinations is scaled such that it can be combined with a second set ofprojection image data generated using an other of these source/detectorcombinations. For example, the projection image data generated using theMV source/detector combination may be scaled such that it can becombined with projection image data generated using the kVsource/detector combination. Following the scaling process, the scaledfirst set of projection image data and the second set of projectionimage data are used to construct a three-dimensional representation ofTarget 210.

In a Generate kV X-Ray Step 310, kV X-Ray Source 120 is used to generatex-rays in the kV energy range. In a Direct kV X-Ray Step 320, these kVx-rays are directed through Target Region 105 in order to image Target210. Target 210 blocks passage of these kV x-rays as a function of theadsorption cross-section of Target 210. In a Detect kV X-Ray Step 330,those kV x-rays that pass through Target 210 are detected using kV X-RayDetector 125. The detection of these x-rays includes generation, andoptionally storage, of projection image data representative of aprojection of Target 210 on kV X-Ray Detector 125.

In a Generate MV X-Ray Step 340, MV X-Ray Source 110 is used to generatex-rays in the MV energy range. In a Direct MV X-Ray Step 350, these MVx-rays are directed through Target Region 105. The MV x-rays can be usedor imaging and/or treatment. In some embodiments, an aperture is used toreduce the volume within Target Region 105 that is exposed to the MVx-rays. While the volume within Target Region 105 that is exposed to kVx-rays and the volume within Target Region 105 that is exposed to MVx-rays are not necessarily the same, these volumes will typicallyoverlap. Some of the MV x-rays are attenuated as they pass through aninstance of Target 210 within Target Region 105. In a Detect MV X-RayStep 360, those MV x-rays that pass through Target 210 are detectedusing MV X-Ray Detector 115. The detected x-rays are used to generateprojection image data representative of a projection of Target 210 on MVX-Ray Detector 115. Detect kV X-Ray Step 330 and Detect MV X-Ray Step360 optionally both include a phase in which projection image data istransferred from kV X-Ray Detector 125 and MV X-Ray Detector 115,respectively, to memory associated with Control Logic 140.

In an optional Move Gantry Step 370, Motor 135 is used to move Gantry130. This movement rotates the kV source/detector combination and/or theMV source/detector combination relative to Target Region 105. Inalternative embodiments, Motor 135 is used to move Target 210 whileGantry 130 remains stationary. Following Move Gantry Step 370, iffurther projection image data is required for the construction of adesired three-dimensional representation of Target 210, then the methodreturns to Generate kV X-Ray Step 310. If sufficient data has beengenerated for the construction of a desired three-dimensionalrepresentation of Target 210 then the method proceeds to a Scale MVX-Ray Data Step 380.

In some embodiments, Steps 310 through 370 are repeated numerous timesin order to generate projection image data at a sufficient number ofdifferent projection angles to generate a desired three-dimensionalrepresentation. A projection angle is the angular position of an x-raysource around Target Region 105 relative to a fixed reference angle. Forexample, an angular position directly above Target Region 105 may beassigned 0 degrees while an angular position directly below TargetRegion 105 is assigned 180 degrees. The larger the number of differentprojection angles the greater the resolution of the three-dimensionalrepresentation, and the longer the imaging process takes. In someembodiments, Gantry 130 is rotated such that both MV and kV projectionimages are generated at overlapping projection angles. Typically, someor all of Steps 310-330 are performed in parallel in time (e.g., attimes that are at least partially overlapping) with Steps 340-360. Forexample, any of Steps 310-330 may be performed parallel in time withGenerate MV X-Ray Step 340. Thus, two different projection images,optionally using two different x-ray energies, can be generated at thesame time. Thus, in some embodiments, kV X-Ray Source 120 and MV X-RaySource 110 are used to generate x-rays simultaneously. In theseembodiments, scatter correction is optionally used to reduce cross-talkbetween each source/detector combination. For example, in oneembodiment, scatter correction is used to reduce the generation of noiseat kV X-Ray Detector 125 resulting from x-rays generated using MV X-RaySource 110 and scattered to kV X-Ray Detector 125 by Target 210.

FIG. 4 is a graphical representation of projection angles used togenerate projection images, in various embodiments of the invention.Relative to a projection angle arbitrarily labeled 0 degrees, kV X-RaySource 120 is rotated to various projection angles between 270 degreesand 10 degrees by moving Gantry 130. At the same time, MV X-Ray Source110, which is fixed at a position on Gantry 130 at a positionapproximately 90 degrees from kV X-Ray Source, is rotated to variousprojection angles between 0 degrees and 100 degrees. Using this rotationscheme, projection images are recorded between projection angles of 0degrees and 10 degrees using both the kV source/detector combination andthe MV source/detector combination. As is discussed further herein, theprojection image data generated at these overlapping projection anglesmay be used to determine scaling factors for converting projection imagedata obtained using x-rays of one energy for combination with projectionimage data obtained using x-rays of the other energy.

Referring again to FIG. 3, in a Scale MV X-Ray Data Step 380 projectionimage data generated using MV X-Ray Detector 115 in Detect MV X-Ray Step360 is scaled such that it can be combined with projection image datagenerated using kV X-Ray Detector 125 in Detect kV X-Ray Step 330. Thisscaling is performed using Conversion Logic 145. Typically, the scalingprocess involves multiplication of the projection image data by ascaling factor or application of a non-linear scaling function. Inalternative embodiments, the kV projection image data is scaled forcombination with the MV projection image data.

In a Construct Image Step 390, CT Construction Logic 150 is used togenerate a three-dimensional representation of Target 210, orcross-section thereof, using both the scaled MV projection image dataand the kV projection image data. The construction of thethree-dimensional representation can be performed using any of the knownalgorithms for generating three-dimensional representations fromtwo-dimensional projections known in the art of computerized tomography.Cross-sections of the three-dimensional representation are optionallydisplayed to a user using Display 155.

FIG. 5 is a scatter plot of kV and MV conversion parameters, accordingto one embodiment of the invention. These data are generated bycomparing projection images obtained at the same projection angles usingx-rays of two different energies, such as kV and MV energies. These sameprojection angles include, for example, the angles between 0 degrees and10 degrees as shown in FIG. 4. Because, in some embodiments, the kVsource/detector combination is oriented at a position approximatelyorthogonal to the MV source/detector combination, Gantry 130 (FIG. 1) isrotated 90 degrees in order for the MV source/detector combination togenerate projection images at the same projection angles as the kVsource/detector combination.

The scatter plot shown in FIG. 5 is generated by examining pixels in theMV projection images and noting the intensity of detected MV x-rays atthose pixels. The corresponding pixels (e.g., the pixels representingthe same positions) in the kV projection images are then examined andthe intensity of the detected kV x-rays at those corresponding pixelsare noted. In some instances, the noted values are normalized bydividing data obtained with an instance of Target 210 placed in TargetRegion 105 by data obtained without any target placed in Target Regions105 (e.g., background data). The kV and MV intensity values are thenplotted. A conversion parameter P for both kV and MV projection imagesis calculated using the formula P=log [(I_(b)−I₀)/(I_(b)−I)]. Where I₀was the pixel value from the open field and I was the pixel value fromthe original projection image. I_(b) was the background pixel value.This formula is based on an assumption that the radiation beam wasattenuated exponentially through the imaging object. In FIG. 5 theplotted intensities have been fitted to a Line 510. Line 510 isoptionally a linear function, in which case the slope and intercept canbe used to convert MV projection image data to kV projection image datain Scale MV X-Ray Data Step 380 of FIG. 3.

FIG. 6 illustrates the conversion of MV projection image data to kVprojection image data using the conversion parameter P calculated asdiscussed with respect to FIG. 5. The data shown represents across-section across a projection image. MV projection image data isrepresented by MV Line 610, kV projection image data is represented bykV Line 620, and converted MV projection image data is represented byConverted Data Line 630. Discrepancies between the kV profile and theconverted MV profile in FIG. 6 are partially attributable to the use ofa linear fit used to determine proportionality constant P. Inalternative embodiments, a non-linear fit is used.

In various embodiments, different approaches are used to determine theconversion parameter P. For example, in some embodiments, projectionimages are acquired using a CT phantom having regions of differentdensity. Because the regions within the CT phantom are wellcharacterized, data taken using kV x-rays and MV x-rays can be comparedtypically at the same projection angle. Once the conversion parameter Pis determined then a Target 210 of interest (e.g., a patient) is placein Target Region 110 and the previously determined conversion parameterP is used to convert the projection image data of the Target 210 ofinterest. In some embodiments, the conversion parameter P is determinedusing a less precisely characterized Target 210 of interest. In theseembodiments, data obtained at overlapping projection angles are used todetermine the conversion parameter P. In some embodiments, both a CTphantom and overlapping projection angles are used to determineconversion parameter P.

In some embodiments, the projection images generated using kV x-rays andMV x-rays are of different dimensions. For example, the volume coveredby MV x-rays may be truncated such that it includes only a subset of thevolume covered by kV x-rays. This arrangement may be desirable when theMV x-rays are used therapeutically and there is a wish to limit theexposure of healthy tissue to MV x-rays. Thus, in some embodiments ofthe invention, a larger target volume is covered by kV x-rays for thepurpose of imaging Target 210 while a smaller target volume is coveredby MV x-rays for the purpose of treatment. Further, those MV x-rays usedfor treatment are optionally also used to enhance the three-dimensionalrepresentation by combining the MV projection image data with the kVprojection image data as described herein. Resolution of thethree-dimensional representation is enhanced in the volume of Target 210receiving therapeutic x-rays.

FIGS. 7A and 7B are CT images obtained using MV and kV imaging,respectively, according to one embodiment of the invention. A CT imageis a cross-section of a three-dimensional object generated usingcomputerized tomography. In this embodiment, projection images wereacquired using a Varian Clinac 21EX accelerator (Varian Medical Systems)as MV X-Ray Source 110 to generate MV x-rays, and amorphous siliconelectronic portal imager (a Si500 detector) as MV X-Ray Detector 115 todetect the generated MV x-rays. A Varian Medical System's On-BoardImager™ (OBI), including both kV X-Ray Source 130 and kV X-Ray Detector125, was used to generate and detect kV x-rays. These systems weremounted orthogonally on Gantry 130 as shown in FIGS. 1 and 2. Threerobotically controlled Exact™ (Varian Medical Systems) supportive armswere used to position MV X-Ray Detector 115, kV X-Ray Detector 125 andkV X-Ray Source 120 such that the beam lines of the MV source/detectorcombination and kV source/detector combination intersected orthogonallynear the center of Target Region 105. The active imaging area for bothkV and MV source/detector combinations was 397×298 mm. The matrix sizefor MV X-Ray Detector 115 was 1024×768 pixels with 2 bytes depth, andthe matrix size for kV X-Ray Detector 125 could be either 2048×1536pixels (high resolution) or 1024×768 (low resolution) pixels with 2bytes depth. A high-performance scatter rejection grid was mounted inthe front of kV X-Ray Detector 125. The x-ray tube of kV X-Ray Source120 had a target angle of 14 degrees and two focal spots: a nominalsmall spot of 0.4 mm and a nominal large spot of 0.8 mm, per IEC(International Electrotechnical Commission) 60336. The heat loadcapacity of this x-ray tube is 600 k heat units, while the heat loadingof the x-ray tube housing is 2M heat units. The x-ray generator, withinkV X-Ray Source 120, had a maximum output of 32 kW. Projection imagesacquired using the kV source/detector combination could be achieved withtwo different modes: digital radiography, both in high or lowresolution, and digital fluoroscopic imaging with a frame rate of 7 or15 frames per second. The kV source/detector combination also had acone-beam CT acquisition mode which could acquire over 650 projectionswithin less than 70 seconds. This mode is used for clinicalapplications.

In order to acquire the CT images shown in FIGS. 7A and 7B, a Target 210was placed within Target Region 105. Target 210 included a head phantomand a contrast phantom taped together. The head phantom (RANDO®anthropomorphic phantom by Phantom Laboratories, Salem, N.Y.) andcontrast phantom (Mini CT QC Phantom Model 76-430, Nuclear Associates,NY) are experimental tools configured to simulate a clinical instance ofTarget 210, such as a patient. The dimensions of the contrast phantomwere six inches in diameter and one inch in thickness. The contrastphantom included insertions of different densities in six 1.125-inchcircular holes. Projection images using the MV X-Ray Source 110 wereacquired using gantry angles starting from 100 degrees to 270 degrees(IEC convention), with an interval of 2 degrees. The projection imagesusing the kV source were acquired using gantry angles starting from 190degrees to 0 degrees. A total of 96 projections were acquired at eachx-ray energy. Because of the orthogonal relationship between of the MVsource/detector combination and the kV source/detector combination, whenMV X-Ray Source 110 was at 0 degrees, kV X-Ray Source 120 was at 270degrees.

FIGS. 8A-8F include several cross-sections of three-dimensionalrepresentations (e.g., CT images) of one instance of Target 210generated using the techniques described herein. FIG. 8A illustrates aCT image generated using 48 different MV projection images obtained atGantry 130 angles between 100 degrees and 6 degrees. FIG. 8B illustratesa CT image generated using 48 different kV projection images obtained atGantry 130 angles between 94 degrees and 0 degrees. FIG. 8C illustratesa CT image generated using both the 48 MV projection images used togenerate FIG. 8A and the 48 kV projection images used to generate FIG.8B. The kV and MV projection images were obtained and combined using,for example, the methods illustrated by FIG. 3.

The CT image of FIG. 8C includes more detail than either the CT imagesof FIG. 8A or 8B. This improved detail is due to the greater number ofprojections used and possibly the different sensitivities of the kV andMV x-rays. For example, the kV projection images show more contrastresolution for soft tissues while the MV images are less susceptible tosome types of interferences.

FIGS. 8D and 8E include CT images. The CT image shown in FIG. 8D wasgenerated using 96 different MV projection images acquired using Gantry130 angles between 100 degrees and 270 degrees, and the CT image shownin FIG. 8E was generated using 96 different kV projections acquiredusing Gantry 130 angles between 190 degrees and 0 degrees. Forcomparison, FIG. 8F includes a diagnostic CT image, reconstructed byusing almost 1000 projections acquired with a Philips AcQsim CTsimulator.

FIGS. 9A-9C illustrate the use of projection images that cover differentvolumes within Target 210. FIGS. 9A and 9B include CT images generatedusing only 12 full kV projection images, and only 12 truncated MVprojection images, respectively. The MV projection images were generatedusing an x-ray beam truncated by an aperture to reduce the volumeexposed to MV x-rays. In some embodiments, the MV x-rays are truncatedto minimize the exposure of tissues outside a treatment volume to MVx-rays.

FIG. 9C includes a CT image generated using a combination of both the kVprojection images used to generate the CT image of FIG. 9A and the MVprojection images used to generate the CT image of FIG. 9B. As a resultof the combination, the center region of the CT image of FIG. 9C isenhanced relative to that shown in FIG. 9A. Thus, MV treatment x-raysare combined with kV imaging x-rays to achieve greater CT image qualityduring treatment. This improvement may be used to refine the treatmentvolume and reduce the exposure of healthy tissue to harmful x-rays.

Several embodiments are specifically illustrated and/or describedherein. However, it will be appreciated that modifications andvariations are covered by the above teachings and within the scope ofthe appended claims without departing from the spirit and intended scopethereof. For example, while MV X-Ray Source 110 and kV X-Ray source 120are used herein as an example, either may be replaced by alternativeradiation sources. In some embodiments both sources are configured togenerate the same types and/or energies of radiation. In someembodiments, the methods discussed herein are used to generatefour-dimensional CT data that includes a time dependentthree-dimensional representation of Target 210. In some alternativeembodiments, projection images generated using a kV source/detectorcombination are used to generate a first three-dimensionalrepresentation, and projection images generated using a MVsource/detector combination are used to generate a secondthree-dimensional representation. The first and second three-dimensionalrepresentations are then combined using CT Construction Logic 150 toform one or more CT images. In these embodiments, Scale MV X-Ray DataStep 380 and Conversion Logic 145 are optional. In some embodiments,more than two source/detector combinations are used to generateprojection images parallel in time. Control Logic 140, Conversion Logic145 and/or CT Construction Logic 150 are each optionally embodied inhardware, firmware, or software stored in memory.

In some embodiments, the projection images generated using one or moreof the source/detector combinations are each one pixel line, e.g.,one-dimensional. These projection images are each representative of theattenuation of x-rays along a line through Target Region 105. In theseembodiments, a plurality of one-dimensional projection images may beused to generate a two-dimensional representation of Target 210, usingthe systems and method of the invention. The adaptation of the systemsand method of the invention to the generation of two-dimensionalrepresentations from one-dimensional projection images would be apparentto a person of ordinary skill in the art.

In some embodiments, MV projections are generated using x-raysconfigured for x-ray treatment of a patient. In these embodiments, acomputerized tomography image constructed using kV projection image dataand MV projection image data may be used for identifying and/or viewingthe treatment volume. In these embodiments, MV x-rays are used for bothtreatment and imaging.

The embodiments discussed herein are illustrative of the presentinvention. As these embodiments of the present invention are describedwith reference to illustrations, various modifications or adaptations ofthe methods and or specific structures described may become apparent tothose skilled in the art. All such modifications, adaptations, orvariations that rely upon the teachings of the present invention, andthrough which these teachings have advanced the art, are considered tobe within the spirit and scope of the present invention. Hence, thesedescriptions and drawings should not be considered in a limiting sense,as it is understood that the present invention is in no way limited toonly the embodiments illustrated.

Exact™ and On-Board Imager™ are registered trademarks of Varian MedicalSystems, Inc.

1. A system comprising: a first x-ray source configured to generatex-rays having energies in a first range; a first imaging x-ray detectorconfigured to detect the x-rays in the first range and to generateresulting first projection image data of a target; a second x-ray sourceconfigured to generate x-rays having energies in a second range, whereinone of the first range and the second range is a MV range; a secondimaging x-ray detector configured to detect the x-rays in the secondrange and to generate resulting second projection image data of thetarget, the target disposed at an intersection of a first axis betweenthe first x-ray source and the first imaging x-ray detector and a secondaxis between the second x-ray source and the second imaging x-raydetector; a conversion logic configured to convert the second projectionimage data to be compatible with the first projection image data and alogic configured to generate a computerized tomography image using thefirst projection and the second projection image data.
 2. The system ofclaim 1, wherein the first energy range is the MV range and the secondenergy range is a kV range.
 3. The system of claim 1, wherein the firstenergy range is a kV range and the second energy range is the MV range.4. The system of claim 1, wherein the first axis is approximatelyorthogonal to the second axis.
 5. The system of claim 1, wherein thefirst imaging x-ray detector and the second imaging x-ray detector areconfigured to operate in parallel in time.
 6. The system of claim 1,further including a motor configured to move the first x-ray source to afirst set of positions and the second x-ray source to a second set ofpositions.
 7. The system of claim 6, wherein the first set of positionsand the second set of positions include some positions in common.
 8. Amethod comprising: generating first x-rays in a first energy range;directing the first x-rays through a target; detecting the first x-raysto produce first projection image data representative of the target;generating second x-rays in a second energy range, wherein the firstenergy range and the second energy range include at least a MV energyrange and a kV energy range; directing the second x-rays through thetarget; detecting the second x-rays to produce second projection imagedata representative of the target, wherein at least one image of each ofthe first projection image data and the second projection image data istaken at same projection angle relative to the target; and constructinga computerized tomography image using the second projection image dataand the first projection image data.
 9. The method of claim 8, whereinat least one of the acts of generating first x-rays and detecting thefirst x-rays is performed in parallel in time with at least one of theacts of generating second x-rays and detecting the second x-rays. 10.The method of claim 8, wherein the act of generating first x-rays andthe act of generating second x-rays are performed parallel in time, andscatter correction is used to reduce noise resulting from scatteredx-rays.
 11. The method of claim 8, wherein the act of generating firstx-rays is performed at a first series of projection angles relative tothe target, and the act of generating second x-rays is performed at asecond series of projection angles relative to the target.
 12. Themethod of claim 11, wherein the first series of projection angles andthe second series of projection angles include at least one projectionangle in common.
 13. The method of claim 8, wherein the first x-rays aretherapeutic, and the first projection image data are used to improveresolution in a computerized tomography image through combination withthe second projection image data.
 14. The method of claim 8, wherein avolume in the target exposed to the first x-rays is smaller than avolume in the target exposed to the second x-rays.
 15. A systemcomprising: means for generating and detecting first x-rays to generatefirst projection image data representative of a target; means forgenerating and detecting second x-rays to generate second projectionimage data representative of the target, the means for generating anddetecting the second x-rays being configured to either generate thesecond x-rays of different energies than the first x-rays, or generateand detect the second x-rays parallel in time with the generation andthe detection of the first x-rays; means for converting the firstprojection image data and the second projection image data to becompatible; and means for generating a CT image using the firstprojection image data and the second projection image data.
 16. Thesystem of claim 15, further including means for identifying a treatmentvolume using a treatment beam.
 17. A system comprising: a first x-raysource configured to generate x-rays having energies in a MV range; afirst imaging x-ray detector configured to detect the generated x-rayshaving energies in the MV range modified by an aperture to generateresulting first projection image data of a target in partial; a secondx-ray source configured to generate x-rays having energies in a kVrange; a second imaging x-ray detector configured to detect the x-raysin the kV range and to generate resulting second projection image dataof the target, wherein the first projection image data and the secondprojection image data are of different dimensions, the target disposedat an intersection of a first axis between the first x-ray source andthe first imaging x-ray detector and a second axis between second x-raysource and the second imaging x-ray detector; and a logic configured togenerate a computerized tomography image using the first projectionimage data and the second projection image data.
 18. A methodcomprising: generating first x-rays in a MV energy range; modifying thefirst x-rays by an aperture, directing the modified first x-rays througha target in partial; detecting the first x-rays to produce firstprojection image data of partial representative of the target;generating second x-rays in a kV energy range; directing the secondx-rays through the target; detecting the second x-rays to produce secondprojection image data representative of the target; constructing acomputerized tomography image using the first projection image data andthe second projection image data.